Means for modifying signal of facsimile reproduction system



J. A, C. YULE Dec. 26, 1961 MEANS FOR MODIFYING SIGNAL OF FACSIMILE REPRODUCTION SYSTEM 2 Sheets-Sheet 1 Original Filed June 14, 1955 HIS ATTORNEYS Dec. ze, 1961 1^- C' YULE 3,014,983

MEANS FOR MODIFYING SIGNAL OF FACSIMILE REPRODUCTION SYSTEM Original Filed June 14, 1955 2 Sheets-Sheet 2 zERo mK Ll l l l I l Ilnl I l l l l I Il E '1;- I: 1 T Z 0 '5 t I- w n s I I m O (l Z E LUZ U 2 n I s n n 5:1 L|M|T|NG|NK m DENsm-Y VALUE i 2 l 0 L o 3 3 O 0 J I JI l IIII l l I l l llll I l i ll Loc SCANNER OUTPUT SIGNAL I i FIGB. J f SHADOW MEDIAN HIGHLSGHT INTENSITY RaeloN INVENTOR JOHN A.C.YULE

H IS ATTORNEYS United States Patent() 3,014,983 MEANS FOR MODIFYING SIGNAL OF FACSIMILE REPRODUCTION SYSTEM John A. C. Yule, Rochester, N.Y., assigner, by mesne assignments, to Time, Incorporated, New York, N.Y., a corporation of New York Original application June 14, 1955, Ser. No. 515,354, now Patent No. 2,932,691, dated Apr. 12, 1960. Divided and this application Aug. 5, 1959, Ser. No. 838,814

`10 Claims. (Cl. 178-5.4)

This invention relates generally to methods and apparatus for a system which develops an electric signal facsimile of an original visual subject to the end of providing a printed or other permanently subsisting reproduction of the subject. More particularly the invention relates to methods and apparatus of the above-noted character which provide for modifying one or more of the facsimile signals in a manner which compensates for loss of detail in the reproduction becauseof a lesser range of intensity in thev reproduction than in the original subject.

This application is a division of my copending U.S; application Serial No. 515,354, filed .lune 14, 1955;.

For a better understanding of the` description to follow; reference is made to the accompanying drawings wherein:

FIG. l is Aa schematic diagram, partly in block form and partly in detail form, Aof a facsimile reproduction system incorporating the present invention;

FIG. 2 is a detailed schematic diagram of one of the blocks of FIG. 1; and p FIG. 3 -is a graph of aid in explaining the nature of the invention.'

Whilethe invention is vof application in a black and white facsimile reproductionv system, the invention was developed in connection with a color facsimile system,l and, hence', will be described in connection with latter type of system. f The description which follows refers to like elements by utilizing a common reference number for the elements, and by further utilizing prime valuesto rdistinguish between like elements. It will be understood, accordingly, that, unless the context otherwise requires, the description concerning one element applies in like manner to other elements in the drawings having the same reference number. j

In FIG. 1, .the reference numeral designates an electro-optical scanner comprised Vof a conventionaloptical color analyzer 11V and the blue, green and red photo-modulators 12, 12' and 12". The analyzer 11 is of awell-known type-which scans successive elemental areas of a colored original subject v(not shown), and which resolves the color of each scanned area into the three additive primary colors, blue, green and red. -rIhis resolution is effected by a splitting up ofthe light derived from a scanned area -intothree light beams 13, 13 and 13 whose respective intensity values are commensurate withthe respective amounts of the three-mentioned additive color components which make up the color of the scanned area.

The blue light beam 13 is received by the photomodulator 12 which may be similar to the type of photomodulator disclosed in the British Patent 630,888 of 1949 to Jones. This type of photo-modulator lcomprises a photomultiplier wherein, as is usual, the anode-cathode 3,014,983 Patented Dec. 26,v 1961 current of the photomultiplier develops a signal having amplitude fluctuations proportional to the intensity variations of the impinging light'beam, but whereinA a high frequency signal' is injected onto one of the dynodes of the photomultiplier. The operational conditions in the photomultiplier produce lan output signal wherein the mentioned fluctuations representing lightintensity have been 'converted into amplitude modulations on a high frequency carrierV signal having twice the frequency of the injected signal.

' In the present instance, the injection signal for the blue photo-modulator 12 is a 75 kc. signal derivedV by a conventional 2:1 frequency divider 15 and a 75 kc. filter 16 from the output of a kc. oscillator 17. The output of blue photo-modulator 12 is, accordingly, a 150 kc. carrier signal having amplitude modulations which vary in proportion to the intensity lvalues of the blue component in successively scanned elemental areas of the original subject. It'will be seen that the green photo-modulator 12' and the red photo-modulatorlZ" provide output signals of lsimilar nature to the output signal ofblue photomodulator 12. These signals at the outputs of photomodulators 12, 12 and 12" are referredto hereafter as the-scanner signals. l The blue `scanner signal is supplied to the input of a blue electric signal channel which may be, for the most part, similar to one of the color channels in a conventional color facsimile reproduction system, but which is represented herein in simplified form as being comprised ofk a signal modifying circuit 20, an exponential compression circuit 21, and a series of further blue channel stages represented by the block 22. The effect of the signal modifying circuit 20 and the exponential compresso'r Z1 upon the scanner signal will be later described in detaillvr I i A n After being operated on by circuits 20, 21, and after being rectified and otherwise considerably modified in thefurther blue channel stages 2.2, theblue color signal excites a yellow glow 'lamp 23 to emit light of an intensity which varies with the amplitude of the exciting signal. The color signals conveyed through the green and red channels respectively excite -in like manner the magenta and-cyan glow lamps 23 and 23". Also, as is conventionaLjthe blue, green and red signals are fed to a black channel 24 which responsively develops a signal which excites a black glow lamp 25.

' The luminous emissions from the four glow lamps vare formed into reproducing light beams (not shown) which scan in synchronism with the scanning action of the analyzer 11. Sheets having photosensiti-ve emulsions thereon are respectively exposed to these reproducing light beams to give four separation negatives. From these negatives are produced corresponding half-toneprinter plates which are thereafter respectively inked with yellow, magenta, cyan and black inkl The final color reproduction is obtained by transferring in'superposition the four ink images on the half-tone plates onto an inkreceiving medium such as white paper.

The yellowjm'agenta and cyan ink colors are denoted subtractiveg primary colors herein by virtue of their association in this :instance with printing inksl which characteristically have a subtractive or light absorbing effect. The subtractive primary colors` quantitatively bear a reciprocal relation to the additive primary colors.

For example, yellow ink appears yellow by absorbing the blue component ofimpinging white light while reecting the red and green components thereof, the last two components together being seen as the color yellow to the human eye. Hence, a large amount of blue in a color on the original is manifested as a low amounty of yellow ink on the printed reproduction. Similar reciprocal relations exist between the amounts of green and red in the original and the respective amounts laid down of the magenta and cyan inks;- In practice, the reciprocal relations just described are obtained in the course of conversion of a negative into a half-tone plate. WhileV the density of a given area on the negative varies directly with the amplitude of the color signal which excites the glow lamp to expose the area, the amount of ink laid down in the corresponding area of the print varies inversely with the density of the negative in the given area.

It follows that the density on the print` of any one of the four-mentioned inks bearsa reciprocal relation to the amplitude of the signal which excites the glow lamp determining the deposition of that particularink. In other words, a change in one sense of the amplitude of the exciting signals results in a change in the opposite sense of the density of the ink responsively laid down.

Because of inherent limitations in the spectral characteristics -of the inks making up the half-tone print, the range of tone intensities on the original subject may be (and usually is) considerably greaterthanthe range of tone intensities available in the print. For example, While the range-of intensities of the original may be as much as 1-l500 in given intensity units, `the reproduction may have an available range of intensities of no more than l-SO in the same intensity units. This difference between the two intensity ranges makes it necessary, if the tonal details of the original are to be` preserved, to provide for reduction of the full range of original tone intensities to the more .limited range of tone intensities available in the reproduction.

To obtain this reduction in tone intensities, the described reproducing process incorporates a method step wherein the color signals are compressed according to the relation:

E2= (E1)n (1) where k1 is a constant, assumed (for the sake 'of simplicity) to have a value of l. Also, since the intensity of the same additive primary color on the reproducedpprint can be considered, by a reasonable assumption, to be pro` portional -to the amplitude of the signal after compression,

the response V2 of the .hum-an eye to the intensity of this reproduced additive primary color can be represented by the expression:

Vzzkg Log E2=L0g E2 Y where k2 is another constant, assumed (for the sake of simplicity) to also have a value of l. f n

The right-hand 'terms of (2) and (3.). can both be ob,-

tained by taking the logarithm of both sides of .(1) to to give the following relation:

As is well known, (4) can be converted into the following expression:

Log E2=n Log El (5) Substituting V1 and V2 from (2) and (3) for, respectively, the left-hand and right-hand terms of (5), there is obtained the expression:

From (6) it is seen that if n is less than l, the brightness to the human eye of an additive primary color on the reproduction will be less thanthat of the parent color on the original, the relation in visual brightness of the two colors being in accordance With'the value ofVn, the exponent of compression. The conclusion followsy that'if, as set forth by (l), the color signals are compressed in accordance with an exponential function whereof the'exponent, n, is of an appropriate value less than l, the'entire range of tonal intensities of the original can be reduced to tit into the more limited intensity range of the reproduction.

In furtherance of so reducing the tonal intensities, it has been the practice heretofore to give the color signals an exponential compression which is of the linear type in the lsense that the exponent of compression, n (while possibly adjustable from one value to another to provide different degrees of compression for ari-amplitude of given value for the signal), remains constant in value as the amplitude ofthe signal undergoing compression varies within the range of amplitudes which the signal mayassume. A linear exponential compression of this sort is represented lby line Iin the graph of FIG. 3 wherein, in accordance with (5), the logarithmic values of the signal before and after compression are represented by the horizontal and vertical coordinates of the graph, and wherein the slope of line I is substantially equal, throughout, to a fixed value for n.

According to the present invention, it has been found that a linear exponential lcompression of the signals does not give an entirely satisfactory form of reproduction of the original subject. The factor which makes linear exponential compression unsatisfactory can be foundvin the relation between the amplitude ofanylinearly compressed color signal and jthe spectral `characteristics of the ink which is laid'down as a function of the colorsignal. -As stated, if the amplitude ofthe signal represents a given additive primary color, the ink responsive laid down is Aof the reciprocal subtractive color. A's the amplitude ofthe signal decreases to represent a lower intensity of additive primary color onthe original subject, the densityl of thel ink increases to absorb more of the additiveI primary color fromlight impinging on the reproduction, or (as ,an alternative mode of explanation) to reflect from thereproduction more light of the subtractive color which is reciprocal to the additive primary color. As, however, the amplitude of the linearly compressedsignal drops to lower and lower values, the accompanying, progressive increase in ink density reaches a point` beyond which a further increase does vnot serve to appreciably increase the absorption of light of additive primary color by the ink, or (as a different aspect 'of the vsame phenomenon) to increase, on the reproduction, the intensity of the reciprocal subtractive color. Thus, any coloredink has a limiting density value beyond which any further ink laid down represents a mere overdepos'it which serves no Vuseful purpose, and which has an unattractive appearance on the print. v

This limitation in the spectral characteristics of the colored inks gives rise to the following problem. -If the ink density is made a function of a color signal whichhas undergone a linearexponential compression whereof Athe exponent n vis of'sufciently low value that the maximum density of ink laid down `does not exceed the mentioned limiting value,"it has Ybeen found, in the median density range for the ink, that the spread Yin Vvisual brightness of the reproduced colors will not be sufficient vto realistically reproduce the corresponding spread` in visual brightness of the parent colors on the original subject. On the other hand, if the ink density is made a function of a color signal which has undergone a linear exponential compression whereof the exponent, n, is of sufficiently high value that a realistic reproduction as to visual brightness is obtained in the median density range, it has been found that the signal will, on occasion, cause the ink density to exceed the mentioned limiting value therefor.

It is accordingly an object of this invention to provide methods and apparatus for compressing a signal, representing visual information on an original subject, in a manner such that the range of reproduced tonal intensities derived from -the signal will, in terms of appearance to the human eyeg'represent most effectively, the full range of tonal intensities of the original subject.

Itis another object of this invention to provide methods and apparatus of the above-noted character for compressing the signal in such manner that an overdeposit of ink is avoided when the reproduction is in the form of an ink print.

These and other objects are realized, according to an apparatus manifestation of the invention, by providing an electric compressor means and a signal modifying means in combination with a facsimile reproduction system wherein an electro-optical scanner develops an electric signal proportional to the varying intensity values of successively scanned areas of an original subject. The compressor means is adapted to compress the output signal from the scanner so that the output signal from the cornpressor means is proportional to the amplitude of the scanner signal raised to an exponent having a value of less than l.l i The signal modifying lmeans is of such nature, and is so connected with the scanner and with the 'compressor means, that the signal modifying means by its operation renders the value of the mentioned exponent a function of the amplitude of the scanner signal. The mentioned objects and other objects are also realized according to the methods of the present invention by developing an electric signal proportional in amplitude to the varyingintensity values of successive elemental areas lon an original visual subject, additively combining this signal with a signal of constant amplitude, and compressing the resultant signal by, a mode of compression which follows an exponential function whereof the exponent is invariable during changes in the amplitude of rthe first-named signal. Fora better understanding of the invention, reference is made to the showingin FIG. l of one `apparatus ernbodirnent suitable for carrying out the method of the invention. This embodiment is comprised, in the blue color channel, of the signal modifying circuit 20' and the exponential compresser circuit l21. Considering first the exponential compressor, this circuit (as shown in FIG. 2) comprises a network made up of the non-linear resistance devices 30, 31 and 32, and of one selected linear resistor from -each of three groups 33m-33d, 34a-34d, 35u- 35d ofilinear resistorsthe selection amongthe various resistors in each group being accomplished by the gang-operated switches 36, 3,7-, 3,8. The devices 30, 31, 32 may be comprised, for example, of silicon carbide resistors usually, known in the `electrical industry as ,Thy'rite resistors. By good engineering practice, the resistance values of the individual linear resistors may be proportioned-relative to .each other and relative to the resistance characteristics of the non-linear devices .30, 31, 32 to provide for different degrees of compression of a signal of a given amplitude which is supplied as an input to the compressor circuit. As an aid to bringing the actual signal compression which takes place into conformity with a preselected mode of compression for the signal, the circuit includes a variable tap resistor 40 which is connected to inject an adjustable D.C. biasing voltage from its tap 41 into the network of non-linear resistance devices and linear resistors. Also, the compressor circuit 21 includes a feature wherein a rectified form o-f the blue color signal is fed fro-m a subsequent point in the blue color channel, through a lead 42, a resistor 43, a parallel tuned circuit 44 (resonant at the frequency of the compressor input signal), to the input of the compressor network. It has been found that such feedback signal increases the effectiveness of the network in compressing the high frequency input signal thereto.

The compressor circuit 21 compresses the high frequency input signal by an exponential compression Whereof the exponent remains invariable during changes in the modulation amplitude of the input signal. If the circuits preceding the compressor 21 were such that the compressor input signal was proportional to the scanner signal in amplitude, the relationship between the compressed signal and the scanner signal would be one of linear exponential compression. In other Words, as shown by line I of the graph in FIG. 3, the logarithm of the modulation amplitude of the compressed signal would be linearly related to the logarithm of the modulation amplitude of the scanner` signal, with the exponent of compression n being the fixed value slope of this linear function. As discussed heretofore, this linear relationship is incompatible with the objective of obtaining the best results in reproduction.

It has been found, according to the invention that for best reproducing results, the relation between the scanner signal and the compressed signal should be the relation represented by line II of the graph of FIG. 3. As indicated by line Il, an attribute of this optimum relation between scanner signal and compressed signal is that the exponent of compression, n, is a function of the amplitude of the scanner signal, rather than being invariable during changes in such amplitude. More specifically, as shown by the variable Value slope (representing n) for line II, the

value of n should decrease with decreasing amplitude ofk the scanner signal in order to obtain the optimum scanner signal-compressed signal relationship,

The described variation in value of the exponent of compression in the scanner signal-compressed signal relation, is provided for, in effect, by the action of the signal modifying circuit 20. In this circuit, the scanner signal is impressed on the control grid. 50 of a pentode 5,1 having a cathode 52 connected 'to ground through a vcathode resistor 53, and having an anode 54 connected to a positive voltage supply (not shown) through an anode-load resistor 55. These connections render the pentode 51 a cathode grounded amplifier. As is seen in FIG. 1, certain ancillary connections are associated with the amplifier. For example, afeedback path is provided from the midpoint of cathode resistor S3 to grid 50 in order to increase the input impedance of the pentode. Also, theanode current for the tube is filtered by acondenser- 5.6 connected between ground and the junction 57 of anode load resistor 55 with-a resistor 58 located between resistor 55 and the positive voltage supply. The screen voltage is obtained in the usual `manner by a resistor 59 connected between the screen 60' and junction 57, and by a filter condenser 61 connected between the screen 60 and the cathode 53.

lAs is evident, the scanner signal which'is supplied yto the grid 50 of pentode 52 will appear in amplified form at the anode 54 of this pentode. It will be recalled that this amplified scanner signal is in the form of modulation on a' 150 kc. carrier.V IConnected by one end to the anode S4 is a current limiting resistor 70 which is connected at the other end to a lead 71which supplies a constant voltage kc. signal from oscillator 17 to all three of the signal modifying circuits 20, 20', 20"'. Since the 150 kc. oscillations of the scanner signal are synchronized in the first instance with those of oscillator 17, the signal on lead 71 will be both co-equal in frequency with and synchronized in phase with the oscillations of the carrier signal.

By conventional phasecontrol means (notshown), the phases of the two signals are so synchronized that the signal injected from lead 7i through resistor 70 onto anode-54 is substantially in phase with the scanner-signal appearing at anode 54.

The current limiting resistor 76 andthe anode load resistor 55 form a-mixing network wherein a constant amount of the injection signal from `lead 71 is additively combined at anode 54 with the varying amplitude scanner signal, Thus, the signal compressed by exponential compressor 2l is not the scanner signal alone, but is, instead, a resultant signal constituted of the scanner signal increased in amplitude by the constant amount contributed by the injection signal. l

vAs. is seen in graph line II (FIG. 3), the addition of the constant amplitude injection signal to the variable amplitude scanner signal has the effect, in the scanner signal-compressed signal relation, of rendering n, the exponent of compression thereof, a function of the scanner signal amplitude. For example, assume that the scanner signal may have an amplitude range of 1-500 units, and that the injection signal adds units to the scanner signal. At the low end of the amplitude scale the amount contributed to the resultant signal by the injection signal will be considerably larger than the mount contributed by -the scanner signal. Accordingly, a certain percentage change in the scanner signal amplitude results in much less of a percentage change in the resultant signal amplitude. This disproportion in percentage change has the result that the scanner signal will appear to be more compressed in its low amplitude region than at higher amplitudes where thev amount contributed by the injection signal to the resultant signal is relatively small compared to the total amplitude o-f the resulant signal.

Translating the above-described scanner signal-compressed signal relationship into terms of the density of the ink responsively laid down, for low amplitude values of the scanner signal (corresponding to high density values for the ink) the density of the ink is somewhat 'more vcompressed than forhigh amplitude values of the signal (corresponding to low density values for the ink). It has been found that this additional compression of the ink density in the high density region permits a visually realistic reproduction of the original colors to be obtained throughout the full ink density range. At the same time, the additional compression in the high density range prevents an increase in ink ydensity beyond the metioned limiting value thereof.

`It will be understood that, while the foregoing description of apparatushas been confined principally to the blue color channel, .the green and red color channels of the described facsimile reproduction system have like apparatus and perform like operations in the course of compressing the green and red color signals.

The embodiment above described being exemplary only, it will be understood that the present invention comprehends embodiments differing in form or in detail from the presently described embodiment. For example, while the invention has been described in connection with a color reproducing system, the invention is also applicable to a black and white reproducing system. Accordingly, the invention is not to be considered as limited save as is consonant with the scope of the following claims.

JI claim:

1. In a facsimile reproduction system wherein an electro-optical scanner is adapted to develop an electric signal proportional in amplitude to the varying intensity values of successively scanned areas of an original subject, the combination therewith comprising, electric compressor means having an output and connected in a common electric signal channel with said scanner to developy at said output a compressed form of said scanner signal, the amplitude of said compressed signal being proportional to the amplitude `of said Ascanner signal raised to an Aexponent, andsignal modifyingmeans connected in circuit between, said scanner and said output 4for producing a variation in ltheLvalue Vof said exponent as .azfunction of the amplitude of said scanner signal 1by.additively come bining with scanner signal a'constant amplitude signal of lesser 'amplitude' than the range of. amplitude variation of the scanner signal combined therewith:

2. A system as in claim `1 wherein the value .of said exponent decreases with a decrease in v.the light intensity represented by said scanner' signal.

3. In a facsimile reproduction system wherein an electro-optical scanner is adapted to develop an electric signal proportional to the varying intensity values of successively scanned areas of an original subject, the combination therewith comprising, electric compressor -means having an output and an input, said compressor means being in a common electric signal channel with said scanner, and said compressor'means being adapted between its input and output` to compress-the signal passing therethrough in accordance with an exponential function whereof the exponent has a value of less than l and is invariable during changes in amplitude of said last named signal, and an electric signal mixing means interposed in said channel between said scanner and said input `of said compressor means foradditively combining in said vchannel with the scanner signal asignal of constant vamplitude having a lesser amplitude than the range of amplitude variation of the scanner signal combined therewith, the resulting signal being Vcompressed by said compression means to provide therefrom an output signal which is .related in amplitude to the scanner signal by an exponential function whereof the exponent yvaries as a function of the amplitude of said scanner signal.

4. In la facsimile reproduction system wherein an electro-optical scanner isadapted to -develop anelectric signal having fluctuations proportional in amplitude to the varying intensity values of successively scanned elemental areas of an original subject, and wherein said lluctuations are converted into .amplitude modulations on a ,high frequency carrier signal, the combination therewith comprising electric compressor means having yan outputand an input .connected in. a common electric signal Vchannel with said scanner, said compressormeans beingv adapted between its input and output Ato compress the signal passing therethrough in accordance with Yan exponential function whereof the exponent Yhas a value of less than l and is invariable during changes in amplitude of said last named signal, electric signal mixing meanshaving an injection input and connected in said channel between said scanner and the input of said compressor means to additvely combine in said channel an amount of -a signal received at said injectioninput with the modulated high frequency scanner signal, and means for supplying to said injection input, a constant amplitude injection .signal synchronized in, phase and co-equal in frequency with the modulated high frequency scanner signal and having a lesser peak amplitude than the range of amplitude variation .in the modulation envelope .of the modulated scanner signal combined therewith, the signal resulting from said two last-named signals being compressed by said compressor meansto provide therefrom. an output in the form of a modulated high frequency signal which is .related in modulation amplitude to the high frequency scanner signalby an exponential function whereof the value of the exponent varies with the modulation amplitude `of the scanner signal.

' 5. In a facsimile 'reproduction system wherein an electro-optical scanner is ladapted to develop an electric signal having lluctuations proportional in amplitude to thevarying intensity values of successively scannedelemental areas of an original subject, and wherein said lluctuations are converted Vinto amplitude modulations on a high frequency carrier signal, the combination therewith comprising electric compressor'meanshaving an output and an input connected in acommon electric signal channel with said scanner, said compressor means being adapted between its input and output to compress the high frequency signal passing therethrough in accordance withv an exponential function whereof the exponent has a value of less than land is invariable 4during changes in amplitude of the last-named signal, an amplifier tube interposed in said channel between said scanner and said compressor means, said amplifier tube having a cathode coupled to ground, a control electrode connected to receive the modulated high frequency scanner signal andan anode manifesting said scanner signal in amplifier form and connected to supply signal to the input of said compressor means, an anode load resistor connected to said anode, a current limiting resistor connected by one end to said anode, and means supplying to the other end of said current limiting resistor -a constant amplitude injection signal synchronized in phase and co-equal in frequency with the high frequency scanner signal, said amplifier, anode load resistor and current limiting resistor forming a mixing network which additively combines a constant amount of said injection signal with said amplified high frequency scanner signal, the resulting signal being compressed by said compressor means to provide therefrom an output in the form of a modulated high frequency signal which is related in modulation amplitude to the high frequency scanner signal by an exponential function whereof the exponent varies with the modulation amplitude of the scanner signal.

6. In a color facsimile reproduction system wherein an electro-optical scanner is adapted to develop a plurality of electric signals respectively proportional in amplitude to the varying intensities of a plurality of primary color components inhering in the colors of successively scanned elemental areas of an original subject, the combination therewith of a plurality of electric signal channels each carrying a respective one of said signals, and each channel comprising, electric compressor means having an output and an input of which the latter is connected to receive the respective channel signal, said compressor means being adapted between its input `and output to compress the signal passing therethrough in accordance with an exponential function whereof the exponent has a value of less than 1 and is invariable during changes in amplitude of the last-named signal, and electric signal mixing means interposed in the respective channel between said scanner and said input of said compressor means for additively combining with the scanner signal a signal of constant amplitude and having a lesser amplitude than the range of amplitude variation of the scanner signal combined therewith, the resulting signal being compressed by said compression means to provide therefrom an output signal which is related in amplitude to the parent scanner signal by -an exponential function whereof the exponent varies with the amplitude of the parent scanner signal.

7. lIn a color facsimile reproduction system wherein an electro-optical scanner is adapted to develop a plurality of electric signals respectively proportional in amplitude to the varying intensities of a plurality of primary color components inhering in the colors of successively scanned areas of an original subject, and wherein the fluctuations of each signal are converted -into modulations on a corresponding high frequency carrier signal, the combination therewith of a plurality of electric signal channels each carrying one of said high :frequency signals and each channel comprising, electric compressor means having an output and -an input, the latter being connected in the respective channel to receive the respective channel signal, said compressor means being adapted between its input and output to compress in amplitude the high frequency signal passing therethrough in accordance with an exponential function whereof the exponent has -a value of less than 1 and is invariable during changes in modulation amplitude of the last-named signal, electric signal mixing means having an injection input and connected in the respective channelbetween said scanner andthe inputof said compressor means, said' mixing means being adapted to additively combine an amount of a signal receivedat said injection input with thehigh frequency scanner signal, said combination further comprising means for supplying to the injection input of each mixing means a constant amplitude injection signal synchronized in phase with and co-equal in frequency with the modulated high frequency scanner signal and having -a lesser rality of electric signals respectively proportional in amplitude to the varying intensities of a plurality of primary color components inhering in the colors of successively scanned areas of an original subject, and wherein the fluctuations of each signal are converted into modulations on a corresponding high frequency carrier signal, the combination therewith of a plurality of electric signal channels each carrying a respective one of said high frequency signals and each channel comprising, electric compressor means having an output and an input, the latter being connected in the channel to receive the respective channel signal, said compressor means being adapted between its input and output to compress in amplitude the high frequency signal passing therethrough in accordance with an exponential function whereof the exponent is invariable during changes in amplitude of the last-named signal, an amplifier tube interposed in the respective channel between said scanner and the compressor means, the amplifier tube having a cathode coupled to ground, a control electrode connected to receive the modulated high frequency scanner signal of the channel and an anode manifesting the scanner signal in amplified form and connected to supply signal to the input of the associated compressor means, an anode load resistor connected to said anode, a current limiting resistor connected by one end to said anode, said combination further comprising means supplying to the other end of each current limiting resistor a constant amplitude injection signal synchronized in phase with and co-equal in frequency with the associated high frequency scanner signal, each `anode load resistor and associated current limiting resistor forming a mixing network which additively combines a constant amount of said injection sigvnal with said scanner signal, the resulting signal being compressed by the associa-ted compressor means to provide therefrom an output signal which is related in modulation amplitude to the scanner signal by an exponential function whereof the exponent varies inversely with the amplitude of the scanner signal 9. A method for electric signal transmission of visual information inhering in an original visual subject comprising, developing an electric signal proportional in amplitude to the varying intensity values of successive elemental areas on said subject, additively combining said signal with a signal of constant amplitude and of lesser amplitude than the range of amplitude variation of said first named signal which is combined therewith, and compressing the resultant signal by a mode of compression which follows an exponential function whereof the exponent has a value of less than l and is invariable during changes in the amplitude of the resultant signal.

10. A method for electric signal transmission of visual information on an original subject comprising, developing an electric signal having iiuctuations proportional in 1'1 12 amplitude to the varying intensity values .of successive an exponential :function whereof the exponent has a value elemental areas'on the .,su-bject, converting the fluctuaof lessY than j1 and fis invariable in valueduring changes tions vofk said signal-into .modulations one highfrequency in :amplituder of the-resultant signal.rv carrier, ,additively combningto the ,high frequency car.-

; l t rier signala 4constant amount vof a :signal in phase with 5 References Cited in the'file 'ofrthis patent and co-equal lin frequency with the carrier signal and UNITED STATES PATENTS 4of 'lessen peak amplitude than the `range of amplitude l v variation in ;the Ymodulationenvelope ofthe modulated 2,581,124 -M Jan* 1, 1952 carriei-signal combined therewith, and compressing the 22730567. MCCQUBCU Ian- 10,1956

resultantsgnal by a-modeof compression which follows 1g 873,312 t -Moe v.. ,.Feb. 17,0, l-959 

